The present invention relates to a saw having an inner hole and adapted particularly for cutting thin disks from bars.
Saws of the type under discussion normally include a thin blade provided in the center line thereof with an inner hole. This blade carries in the edge region of the inner hole a coating made of diamond or boron nitride grains which are held in a galvanic bonding. The coating covers the internal cylindrical face of the inner hole and the edge portion of the blade at two external sides thereof.
Such saws make use, for example, in the electronic industry for cutting "wafers" which are extremely thin disks cut from the bar of monocrystalline silicon. A particular utilization of such saws can be found for making chips.
The saw having an inner hole is usually formed of a laminated sheet or blade which has the thickness of, for example 0.12 mm or 120 .mu.m. This sheet or blade has an inner hole the edge of which at all sides is covered with a diamond coating galvanically applied thereto. The coating normally covers the inner edge of the hole so as to form a bead at the end of the blade. In addition to this coating on the inner cylindrical surface of the hole the blade also has at two sides thereof, namely at two edge regions closing the hole a diamond coating of about 2 mm in length.
When such saws with an inner hole are used the thin sheet or blade is clamped in a clamping ring to provide high rigidity. The clamping ring is positioned in the saw machine and is rotated when in operation. Thereby the distance between the inner edge of the inner hole of the saw blade and the clamping ring permits the cutting of the bar of the corresponding diameter. The use of such a saw is particularly advantageous when it is necessary to cut articles of very expensive materials such as monocrystalline silicon because the actual width of the cut and assumed losses should be small.
The problem with these otherwise satisfactory saws provided with inner holes is however that during the cutting process the disk being cut could bend towards the saw blade. To reduce such a bending the saw blade during the cutting process has been positioned with its stable side to face the bar and with its non-stable side to face the disk being separated from the bar. It should be noted in this connection that due to relatively large forces on the stable side no elastic or plastic deformations occur. Relatively small forces on the non-stable side of the blade can, however, cause, particularly in dependence upon the thickness of the disk being cut off the bar elastic and also plastic deformations lead in unfavorable cases, to a strong bending of the disk being cut off towards the saw blade or even to the contact with the saw blade. Due to the occurring friction such a bending can cause a destruction of the disk due to the deformations in the disk and losses of its stability. To avoid such a disadvantage it has been proposed to provide a thicker diamond coating at both sides of the saw blade. If the bead-shaped diamond coating at both sides of the saw blade were substantially reinforced this would result in a considerably greater width of the cut which is disadvantageous taking into consideration the fact that this would lead to losses of an expensive material.